docs: def tag_arxiv(line):
      """Tag arxiv report numbers
  
      We handle arXiv in 2 ways:
      * starting with arXiv:1022.1111
      * this format exactly 9999.9999
      We also format the output to the standard arxiv notation:
      * arXiv:2007.12.1111
      * arXiv:2007.12.1111v2
      """
      def tagger(match):
          groups = match.groupdict()
          if match.group('suffix'):
              groups['suffix'] = ' ' + groups['suffix']
          else:
              groups['suffix'] = ''
          return u'<cds.REPORTNUMBER>arXiv:%(year)s'\
              u'%(month)s.%(num)s%(suffix)s' \
              u'</cds.REPORTNUMBER>' % groups
  
      line = re_arxiv_5digits.sub(tagger, line)
      line = re_arxiv.sub(tagger, line)
      line = re_new_arxiv_5digits.sub(tagger, line)
      line = re_new_arxiv.sub(tagger, line)
      return line